The invention relates to photovoltaic solar cell arrays, and in particular, to photovoltaic solar cell arrays having means for minimizing voltage/plasma interaction effects in a space environment.
Spacecraft power levels have continuously increased since the inception of the Space Age, and this trend is projected to continue into the future. Solar arrays producing hundreds of kilowatts are expected to be required by the mid 1990's, and higher power levels will require voltage levels of hundreds to thousands of volts. However, unprotected solar arrays cannot operate at these very high voltages in the space/plasma environment because of the interaction between the solar array and the space plasma. This interaction can cause corona leakage leading to arc discharges, material damage, and the disruption of spacecraft electrical power.
The natural plasma environment is comprised of charged particles in the magnetosphere and the ionosphere. The shape of the magnetosphere is a function of the sun's magnetic field and the earth's magnetic field as shown in FIG. 1. Van Allen radiation belts are an integral part of the magnetosphere and include an electron belt and a proton belt. These radiation belts overlap-the peak density of the electron belt being at an altitude near 26,000 km and the proton belt near 4,800 km. The magnetosphere is supplied with ions from the solar wind which varies as a function of sun spots and solar flares. Variations in plasma density from the solar activity are primarily evident only above the high-density zone of the inner Van Allen electron belt.
Charged particles of the ionosphere are significantly different than those associated with the magnetosphere. Ionospheric ions have very low velocities (low energy) compared to the high energy protons and electrons of the magnetosphere. Consequently, their effect on solar array components is significantly different. High energy plasma particles degrade solar array performance by penetrating through cover and cell material to cause lattice defects deep within the crystalline structure of solar cells. High energy particles travel too fast to be affected by the electric fields associated with the spacecraft. In contrast, low energy plasma particles cannot cause crystalline lattice degradation, but can be affected by local electric fields associated with nearby hardware.
In addition, spacecraft are typically surrounded by a cloud of outgassing products and effluents. Such a cloud becomes a plasma from ionization action caused by galactic cosmic rays and solar photons, just as in the ionosphere. The induced environment also includes radiated electric fields from on-board equipment, and these fields could be strong at the voltage levels required for high-power solar arrays.
A solar array operating in a plasma environment is surrounded by an enhanced electric field region or magnetic sheath. The electric field acts as an arcing path between the solar array and the plasma. The electric field thickness may be obtained from the Child-Langmuir theory: EQU d=9330 {(T).sup.-0.25 (n.sub.e) V .sup.0.75 } (1)
where d is the electric field thickness in meters, n.sub.e is the plasma density in ions/cubic meter, V is the surface potential in volts, and T is the plasma temperature in eV. Corona leakage may lead to arcing on portions of the solar array that operate at negative potentials with respect to plasma ground.
Corona leakage phenomena take place when the potential difference exceeds the breakdown initiation voltage as shown in FIG. 2. Since breakdown initiation voltage varies with solar activity, separate curves are drawn for maximum and minimum levels of solar activity. Arcing causes current surges that produce out of tolerance bus regulation conditions. Material damage can result from carbon tracking and shorting between insulation punctures. Such problems are known to have happened on some spacecraft, and are suspected as the source of anomalies on other vehicles.
In addition, plasma particle current collection causes parasitic losses such as those described in FIG. 3. In this connection, reference is made to the publication of John R. Barton et al., "High Voltage Solar Array Plasma Protection Techniques," Proceedings of the 18th IEEE Photovoltaic Specialist Conference, October, 1985, pp 411-417. Exposed conductors such as solar cell interconnectors act like electrodes in a plasma environment, necessitating their isolation from the plasma.
The plasma interaction problem, which occurs primarily at low and medium altitudes, could be eliminated by keeping the solar array voltage below the voltage at which the leakage is initiated. To avoid plasma interaction when operating a high power electrical system, the solar array voltage must be limited to a safe level and the system voltage is raised by power conditioning to the required level prior to transmission to the electrical loads. The power conditioning can be designed for operation in a plasma environment by insulating exposed terminals and conductors and proper enclosure packaging. Power conditioning, however, introduces power losses of several percent. It is desirable, therefore, to generate high voltage levels directly at the solar array to minimize power conditioning. Such high voltage designs cannot be employed unless plasma interaction is eliminated by protecting the solar array from the plasma.